1. Field of the Invention
The present invention generally relates to a method for optically recording information and more particularly a method for optically recording information in which a multi level signal is recorded on a track.
2. Description of the Related Art The Japanese Laid-Open Patent Application No. 8-147695 (a prior art method {circle around (1)}) discloses such kind of technology. In the prior art method {circle around (1)}, although the power of a laser diode is not decided in an ALPC zone, the laser diode is driven by a simple low bandwidth drive circuit with a multiple level drive current. First, the laser diode is driven on a trial basis in a Test Zone or a Manufacturer Zone, which zones are placed at an inside and an outside of an optical disc, and each zone has more than 100 tracks. In these zones, first, the laser diode is driven by the drive circuit having a current level of Iw0. Next, the laser diode is driven by the drive circuit having a current level of Iw1. Then, the laser diode is driven by the drive circuit having a current level of Iw2. Then, each current level of Iw0, Iw1 and Iw2 is optimally determined for predetermined power levels, respectively. When information is written on the disc in a write mode, first, a w0 D/A converter is supplied with data for the current value Iw0, a w1 D/A converter is supplied with data for the current value Iw1 and a w2 D/A converter is supplied with data for the current value Iw2. Then, switches SW4, SW5 and SW6 are turned on or turned off according to the data to be written. As a result, the laser diode LD1 is supplied with the LD drive current.
The Japanese Laid-Open Patent Application No. 11-25456 (a prior art method {circle around (2)}) also discloses such kind of technology. The prior art method {circle around (2)} discloses a simple method for recording multi level information on an optical information medium in order to raise recording density. In the prior art method {circle around (2)}, initially, first pits are formed on a re-writable media (a phase change optical disc) by radiating a laser light. Then, second pits are formed on the first pits by erasing parts of the first pits according to a multi leveled signal of the information to be-recorded. Sizes of the second pits are modulated according to the multi level information.
FIG. 1 shows an example of a waveform of a recording pulse signal according to the prior art method {circle around (1)} as described above. The waveform of the recording pulse signal is composed of a rectangular recording pulse and an erasing pulse. A means for adjusting the size of a recorded mark controls the width of the recording pulse (T3-T1 as shown in FIG. 1) or strength of a light (Pw as shown in FIG. 1). On reproducing the recorded marks, strength of a reflected light by the mark is varied according to the size of the recorded mark. Therefore, a multi level signal, which is converted from the light reflected and has one of MARK signal levels corresponding to the size of the recorded mark, can be obtained as shown in FIG. 2. FIG. 2 shows a principle of such multi level recording method using a recorded mark size modulation.
A disadvantage of this method using the waveform of the recording pulse signal composed of the rectangular recording pulse and the erasing pulse is that the recorded mark is apt to be enlarged in the radial direction because of thermal diffusion caused by the laser beam focused on the optical information medium. Therefore, the shape of the recorded mark is apt to have a shape like a drop of water as shown in FIG. 1. Therefore, this method can not control the influence of the thermal diffusion on the size of the recorded mark if the size of the mark smaller than the size of the focused laser beam spot is formed.
FIG. 3 shows a relation between an occupied mark ratio and a multi level mark signal level according to the prior art method {circle around (1)}. As shown in FIG. 3, at a region {circle around (12)} in which the mark signal level is relatively high, a ratio of the mark signal level variation to the occupied mark ratio variation is high compared to that at a region {circle around (2)} in which the mark signal level is relatively low. Therefore, there is a need to control the occupied mark ratio more precisely at the region {circle around (1)} than at the region {circle around (2)}. However, the prior art method {circle around (1)} as shown in FIG. 1 cannot control the occupied mark ratio accurately because it cannot control the influence of the thermal diffusion to the size of the recorded mark. Therefore, the prior art method {circle around (1)} cannot control each level of the multi level signal accurately.
FIG. 5 shows the prior art method {circle around (2)}. To eliminate the disadvantage of the prior art method {circle around (1)}, it is possible to use a recording pulse signal composed of multi pulses as shown in FIG. 5. In this prior art method {circle around (2)}, the recording pulse is composed of a start pulse, short multi pulses and an erasing pulse. As the short multi pulses are used, the enlargement of the recorded mark in the radial direction caused by thermal diffusion can be prevented. A means for adjusting the size of the recorded mark controls the width of the first pulse (T2-T1 as shown in FIG. 5), the duty of the multi pulses (T4-T3 as shown in FIG. 5), or strength of the first pulse and the multi pulses (Pw as shown in FIG. 5).
There is a need to raise the frequency of the recording pulses to form the recorded marks having a size less than that of the focused laser beam spot. Therefore, a recording pulse generation circuit and a laser diode need to be operated at a high frequency in order to record the information on the optical information medium at a high speed. However, it is hard to raise their operating frequencies. Further, the recording pulse generation circuit becomes complex.
It is a general object of the present invention to provide a method for optically recording information in which the above disadvantages are eliminated.
Further, it is another object of the present invention to provide a method for optically recording information in which recorded marks having a size less than that of a focused laser beam spot can be formed using a multi level signal recording method with a recording pulse having a simple waveform.
Further, it is another object of the present invention to provide a method for optically recording information in which the size of each recorded mark having a size less than that of a focused laser beam spot can be controlled to a desired size.
Further, it is another object of the present invention to provide a method for optically recording information in which a position of the recorded mark having a size less than that of a focused laser beam spot is controlled precisely by a simple method for controlling the recording pulse.
Further, it is another object of the present invention to provide a method for optically recording information in which a size of the recorded mark having the size less than that of a focused laser beam spot is fine-tuned accurately.
Further, it is another object of the present invention to provide a method for optically recording information in which the position of the recorded mark having a size less than that of a focused laser beam spot is fine-tuned accurately.
Further, it is another object of the present invention to provide a method for optically recording information in which each level of the multi level signal is recorded accurately.
The above objects of the present invention are achieved by a method for optically recording information on an optically re-writable information medium having at least one track, in which method a laser light is focused to a laser spot to form a mark on the optically re-writable information medium and the size of the mark is modulated according to a level of a multi-level signal which correspond to the information to be recorded, further in which method:
the strength of the laser light for recording each mark is modulated according to a waveform comprising a first rectangular erasing pulse signal, a rectangular recording pulse signal, an off-pulse signal and a second rectangular erasing pulse signal,
the laser light having a strength set to a value indicated by the first rectangular erasing pulse signal can erase a recorded mark,
the laser light having a strength set to a value indicated by the rectangular recording pulse signal can record a mark,
the strength of the laser light set to a value indicated by the off-pulse signal is less than the strength of the laser light used in reproduction of the recorded marks,
the laser light having a strength set to a value indicated by the second rectangular erasing pulse signal can erase the recorded mark, and
a product of the time interval of the rectangular recording pulse signal and the relative linear velocity between the laser spot and the optically re-writable information medium is shorter than the length of the recorded mark.
According to the present invention, the mark is formed on a recording medium by radiating the laser light having the strength of the short recording pulse signal followed by the off-pulse signal. The recording film is heated to form the mark and the mark is extended by a thermal diffusion while the off-pulse signal is provided. Therefore, the length of the recorded mark ML is longer than the recording pulse width at the predetermined linear velocity. The erasing pulse signal follows the off-pulse signal and the extended recorded mark is erased by the erasing pulse signal.
The recording method according to the present invention can form a mark on the recording medium by radiating the laser light having a shorter recording pulse width than a recording pulse width used in the conventional method to form a mark having the same length. Therefore, heat quantity applied to the medium by the recording method according to the present invention is smaller than that of the conventional method. As a result, expansion of the mark in a direction perpendicular to the longitudinal direction of the track is reduced compared to the conventional methods.
The above objects of the present invention are achieved by a method for optically recording information on an optically re-writable information medium, wherein the size of the mark is modulated according to the information by means of modulating a pulse width of the off-pulse signal so that the time interval between a rising edge of the rectangular recording pulse signal and a rising edge of the rectangular erasing pulse signal corresponds to the size of the mark.
According to the present invention, a simple recording pulse signal generation circuit can be used for multi-level recording because the size of the mark is modulated according to the information by means of modulating only the pulse width of the off-pulse signal.
The above objects of the present invention are achieved by a method for optically recording information on an optically re-writable information medium, wherein the track is divided into a plurality of cells and the mark is recorded in the cell so that the center of the mark is placed at the center of the cell by means of adjusting both the rising edge of the rectangular recording pulse signal and the rising edge of the rectangular erasing pulse signal.
According to the invention, the center of the mark is precisely placed at the center of the cell by means of adjusting both the rising edge of the rectangular recording pulse signal and the rising edge of the rectangular erasing pulse signal because the effect of the thermal diffusion is effectively used.
The above objects of the present invention are achieved by a method for optically recording information on an optically re-writable information medium, wherein the size of the mark is fine-adjusted by controlling the timing of a falling edge of the rectangular recording pulse signal.
According to the invention, the size of the mark is fine-adjusted by controlling only the timing of the falling edge of the rectangular recording pulse signal because the effect of the thermal diffusion is effectively used.
The above objects of the present invention are achieved by a method for optically recording information on an optically re-writable information medium, wherein the size of the mark is fine-adjusted by controlling the strength of the laser light modulated by the rectangular recording pulse signal.
According to the present invention, the size of the mark is fine-adjusted by controlling only the strength of the laser light modulated by the rectangular recording pulse signal because the effect of the thermal diffusion is-effectively used.
The above objects of the present invention are achieved by a method for optically recording information on an optically re-writable information medium, wherein the timing of the rising edge of the rectangular recording pulse signal is controlled so that the center of the mark is placed at the center of the cell.
According to the invention, the center of the mark is precisely placed at the center of the cell by only further shifting the rectangular recording pulse signal because the effect of the thermal diffusion is effectively used.
The above objects of the present invention are achieved by a method for optically recording information on an optically re-writable information medium, wherein the relation between the length (ML) of the mark along the track and a diameter (BD) of the laser spot satisfies MLxe2x89xa6BD, and
the product of the pulse width of the rectangular recording pulse signal and the relative linear velocity is less than 20% of the diameter (BD) of the laser spot.
According to the present invention, the width of the rectangular recording pulse signal is properly limited so that the mark can be recorded with higher precision.
The above objects of the present invention are achieved by a method for optically recording information on an optically re-writable information medium, wherein the relation between the length of a minimum mark (MLmin) along the track, except when there is no mark, and the diameter (BD) of the laser spot satisfies MLmin/BDxe2x89xa70.10, and
each level of the multi-level signal is assigned so that the level difference between adjacent levels becomes equidistant, except when there is no mark.
According to the present invention, the deviation from the aimed multi-level signal levels can be reduced.
The above objects of the present invention are achieved by a method for optically recording information on an optically re-writable information medium, wherein
the relation between the length of a maximum mark (MLmax) along the track, except for a mark that has about the same length as a diameter of the laser spot, and the diameter (BD) of the laser spot satisfies MLmax/BDxe2x89xa60.70, and
each occupied mark ratio of the multi-level signal is assigned so that the difference between the occupied mark ratios corresponding to adjacent levels of the multi-level signal becomes equidistant, except for the mark that has about the same length as the diameter of the laser spot.
According to the present invention, the deviation from the aimed multi-level signal levels can be reduced.